New Acyclic (π-Allyl)-closo-rhodacarboranes with an Agostic CH₃···Rh Bonding Interaction That Operate as Unmodified Rhodium-Based Catalysts for Alkene Hydroformylation

A series of new agostic (CH₃···Rh) (π-allyl)-closo-rhodacarboranes (π-allyl = 1,1-dimethylallyl, 1,2-dimethylallyl, 1,1,2-trimethylallyl, 1,2,3-trimethylallyl), stable in the solid state, have been synthesized via one-pot reactions between the K⁺ salts of the [7-R-8-R′-7,8-nido-C₂B₉H₁₀]⁻ monoanions (1a, R = R′ = Me; 1b, R,R′ = μ-(o-xylylene); 1c, R,R′ = μ-(CH₂)₃) and the di-μ-chloro cyclooctene rhodium dimer [(η²-C₈H₁₄)₄Rh₂(μ-Cl)₂] (2) in the presence of a 3-fold excess of the conjugated 1,3-dienes 2-methylbuta-1,3-diene (isoprene, 3), 2,3-dimethylbuta-1,3-diene (4), and 3-methylpenta-1,3-diene (5). The agostic structures of [3-{(1–3-η³)-1,1-dimethylallyl}-1,2-(CH₃)₂-3,1,2-closo-RhC₂B₉H₉] (7a) and [3-{(1–3-η³)-1,1,2-trimethylallyl}-1,2-(CH₃)₂-3,1,2-closo-RhC₂B₉H₉] (8a) have been unambiguously confirmed by single-crystal X-ray diffraction studies. Two of these π-allyl complexes prepared were evaluated for their efficacy in hydroformylation of the model alkenes under syngas (CO/H₂) using supercritical carbon dioxide (scCO₂) as the solvent, and both display excellent conversion and high regioselectivity in the formation of aldehyde products.